Trimethylolpropane is a trihydric alcohol which is of significance for the production of coating materials, polyurethanes and polyesters, for example of alkyd resins. Trimethylolpropane is produced industrially by condensation reaction of n-butyraldehyde with formaldehyde according to different variants.
In what is called the hydrogenation process, at least two moles of formaldehyde are added onto one mole of n-butyraldehyde in the presence of a catalytic amount of a tertiary amine via the monomethylolbutyraldehyde intermediate to initially give dimethylolbutyraldehyde, which is then converted to trimethylolpropane in a hydrogenation step. According to the process described in WO98/28253 A1, formaldehyde is used with an up to eight-fold molar excess. The reaction mixture obtained from the aldol addition step is worked up either by distillation or by phase separation. In the distillative workup, unconverted or partly converted starting compounds are drawn off as volatile components and recycled into the reaction stage, while the bottom product is converted further. If, instead of the distillative workup, the reaction mixture is separated in a phase separator into the aqueous and organic phases, the organic phase is returned to the aldol addition and the aqueous phase is processed further. There follows a catalytic and/or thermal treatment in order to convert monomethylolbutyraldehyde to dimethylolbutyraldehyde. By-products formed are removed by distillation and the bottom product of this distillation is subsequently catalytically hydrogenated to obtain trimethylolpropane. The crude trimethylol-propane obtained is subsequently subjected to a purifying distillation. After removal of low and medium boilers, purified trimethylolpropane is obtained as an intermediate fraction, while higher-boiling condensation products within which trimethylolpropane equivalents are bound are obtained as the tailings or bottom fraction.
In addition to the hydrogenation process, trimethylol-propane is also prepared industrially by what is known as the Cannizzaro reaction. In a first reaction stage, n-butyraldehyde and formaldehyde are reacted with addition of stoichiometric amounts of a base to give dimethylolbutyraldehyde, which is subsequently reduced with excess formaldehyde to give trimethylolpropane, while one equivalent of formate is formed simultaneously. Typically, the base used is an aqueous solution of an alkali metal or alkaline earth metal compound, for example sodium hydroxide, potassium hydroxide or calcium hydroxide. Since one equivalent of alkali metal or alkaline earth metal formate is obtained as a coproduct in the Cannizzaro process, the economic viability of this process variant also depends on the marketing opportunities for this coproduct. The workup of the aqueous reaction solution obtained, which comprises trimethylolpropane, alkali metal or alkaline earth metal formate and excess base, is effected generally by extraction. After neutralization of the excess base, the aqueous solution is extracted with an organic solvent, for example with ethyl acetate. The organic phase is separated from the aqueous phase, which comprises the alkali metal or alkaline earth metal formates in dissolved form, and, after removal of the extractant, trimethylolpropane is obtained by distillation. The resulting trimethylolpropane can be subjected to further purification processes. According to U.S. Pat. No. 5,603,835, an aqueous solution is first prepared from resulting trimethylolpropane, and is extracted once again with an organic solvent, for example with methyl tert-butyl ether. Trimethylolpropane is obtained from the resulting aqueous solution with an improved colour number of less than 100 APHA units.
According to the process known from U.S. Pat. No. 5,948,943, the aqueous, crude reaction solution obtained after the Cannizzaro reaction is treated with a suitable organic solvent at such a temperature that only one liquid phase leaves the extraction vessel. In the subsequent cooling outside the extraction vessel, the aqueous phase separates from the organic phase, and trimethylolpropane can be isolated from the aqueous phase with a colour number of less than 100 APHA.
It is likewise known that the Cannizzaro reaction can be performed with an organic base, for example with a tertiary amine. According to the procedure known from WO97/17313 A1, formaldehyde is prepared with n-butyraldehyde in the presence of stoichiometric amounts of a tertiary amine, forming one equivalent of ammonium formate. Subsequently, water, excess tertiary amine and excess formaldehyde are removed from the crude mixture, and the remaining mixture is heated. This dissociates the ammonium formates to the tertiary amine and formic acid, and the tertiary amine and further volatile constituents are removed, resulting in the formation of trimethylolpropane formate. The tertiary amine removed is either recycled into the Cannizzaro stage or used as a catalyst for the transesterification of the trimethylolpropane formate in a downstream reaction with an added lower aliphatic alcohol. The trimethylolpropane released is subsequently isolated from the crude product.
Irrespective of whether the preparation of trimethylol-propane is effected by the hydrogenation process using catalytic amounts of a tertiary amine, by the Cannizzaro process with molar amounts of a tertiary amine and subsequent transesterification of the trimethylolpropane formate formed, or by the Cannizzaro process with molar amounts of alkali metal or alkaline earth metal hydroxides and the extractive removal thereof, the crude trimethylolpropane obtained is subjected to a single or multiple distillative purification, which is effected under reduced pressure due to the high boiling point. According to DE 100 58 303 A1, the distillative workup of the trimethylolpropane is combined with an ion exchanger treatment, in which case either the aldolization output or the hydrogenation output is contacted with a strongly basic ion exchanger before the distillative workup.
Distillative workup gives rise to high-boiling fractions with a higher boiling point compared to trimethylolpropane, or residues in which derivatives of trimethylolpropane are present and have formed therefrom by reaction with, for example, methanol, formaldehyde or else with a further molecule of trimethylolpropane in the upstream reactions. Among these derivatives, particularly formaldehyde-containing acetals are represented, which are characterized by the structural element —O—CH2—O— and can also be regarded as formals. Among the formals, the following linear and cyclic formals of trimethylolpropane can be described structurally:
Monocyclic formal of trimethylolpropane:

Linear bistrimethylolpropane formal:[C2H5C(CH2OH)2CH2O]2CH2  Formula II
Methyl (monolinear) formal of trimethylolpropane:C2H5C(CH2OH)2CH2OCH2OCH3  Formula III
Methyl (bislinear) formal of trimethylolpropane:C2H5C(CH2OH)2CH2OCH2OCH2OCH3  Formula IV
In this context, the monocyclic formal of trimethylol-propane (I) boils at a lower temperature than trimethylolpropane itself. The methanol-derived formals (III) and (IV) have a boiling point comparable to trimethylolpropane, while the linear bistrimethylol-propane formal (formula II) is present as a high-boiling component. In addition, further linear and cyclic oxygen compounds, such as the cyclic formal of ditrimethylolpropane
are present in the distillation residues.
Likewise present in the high-boiling fractions and residues of the distillative workup of crude trimethylolpropane are also substantial amounts of ditrimethylolpropane [C2H5C(CH2OH)2—CH2—]2—O and trimethylolpropane itself. Additionally present in small amounts are low-boiling components, such as methanol or 2-ethyl-1,3-propanediol.
Since the high-boiling fractions and residues of the distillative workup of trimethylolpropane include considerable amounts of derivatives in which equivalents of trimethylolpropane are chemically bound, a number of processes are proposed to dissociate especially formaldehyde-containing acetals and to release trimethylolpropane, in order in this way to improve the yield of the overall trimethylolpropane preparation process. According to WO 2004/013074 A1, the high-boiling fractions and distillation residues obtained in the trimethylolpropane preparation are treated with acid, and the water content in the reaction mixture should be 20-90% by weight. It is possible either to obtain trimethylolpropane by distillation from the acid-treated product or to recycle the treated product into the hydrogenation stage of dimethylolbutyraldehyde to give trimethylol-propane. The hydrogenating dissociation of linear or cyclic acetals in aqueous solutions in the presence of a heterogeneous hydrogenation catalyst to give the desired polyhydric alcohol is known from DE 198 40 276 A1. The process requires hydrogenation temperatures above 160° C. in order to suppress the harmful influence of formates, which may still be present particularly in the case of working by the Cannizzaro process, on the hydrogenation performance of the catalyst. The hydrogenating, catalytic dissociation can likewise be performed in the presence of an acid, for example in the presence of a lower carboxylic acid or acidic solids.
The high-boiling fractions and the residues of the distillative workup of the trimethylolpropane preparation comprise, in addition to the aforementioned formaldehyde-containing acetals, also significant amounts of ditrimethylolpropane, which is likewise of industrial significance as a valuable starting material for production of alkyd resins, plasticizers and lubricants. The prior art discloses processes for obtaining ditrimethylolpropane from these residues, and further purifying product thus obtained if required.
According to DE 2058518 A1, the ditrimethylolpropane-containing distillation residue is subjected to a steam distillation with superheated steam under reduced pressure. After removal of water, ditrimethylolpropane is obtained from the resulting aqueous distillate, and can be recrystallized if required from an organic solvent, for example acetone.
Since the distillative purification of ditrimethylol-propane is possible only with very great difficulty owing to the high boiling point, and there is also a risk of decomposition of the ditrimethylolpropane due to the high temperatures to be employed, the direct workup of the distillation residue by recrystallization to obtain ditrimethylolpropane is also described. DE 2358297 A1 considers the simple crystallization of an aqueous solution of the distillation residue, wherein the salt concentration in the aqueous solution is adjusted to a particular ratio in order to enable the precipitation of ditrimethylolpropane in sufficient purity. When trimethylolpropane is prepared by the Cannizzaro process, the salt content, for example the alkali metal formate content, in the distillation residue may already be sufficiently high to ensure the precipitation of ditrimethylolpropane crystals in a satisfactory manner after dissolution in water. It may be necessary to add a further salt to the aqueous solution, for example an alkali metal salt.
US 2004/0254405 A1 discloses a process for recrystallizing the distillation residue using organic solvents, for example acetone or methyl ethyl ketone, which requires a particular degree of observance of the crystallization temperature, the amount of solvent and the ditrimethylolpropane content in the distillation residue. The use of a mixture of a suitable solvent and water for the isolation of ditrimethylolpropane from the distillation residues of the trimethylolpropane preparation is described in DE 10 2008 038 021 A1. An organic solvent phase and a viscous residue are initially obtained, the phases are separated and the organic solvent phase is extracted with water. The water phase is isolated and solvent residues present are removed. Ditrimethylolpropane is crystallized from the remaining water phase.
The known processes for obtaining ditrimethylolpropane from high-boiling fractions and residues which have a higher boiling point than trimethylolpropane and which are obtained in the distillative workup in the course of trimethylolpropane preparation require either complex recrystallization steps or a complex steam distillation with the subsequent removal of water from the steam distillate.
There is therefore a need to obtain ditrimethylol-propane from such high-boiling fractions and residues in a very simple manner with such a purity required for the envisaged industrial applications. At the same time, trimethylolpropane still present in a physical mixture in these fractions and residues, and also derivatives present therein containing chemically bound trimethylolpropane units, should likewise be isolated as a trimethylolpropane-rich fraction which can be recycled back into the trimethylolpropane purification process, such that not only the recovery of pure ditrimethylolpropane but also the yield of trimethylol-propane over the entire preparation process can be improved. In this way, the high-boiling fractions and residues which are obtained in the distillative workup in the course of trimethylolpropane preparation can be utilized in a very economically viable manner.